The present invention relates to a CMI (Coded Mark Inversion) block synchronization circuit capable of easily preserving block synchronization not only for random errors of inputted CMI codes but also for burst errors.
A CMI code has the configuration that one bit of a binary code is assigned two bits. A binary code "1" is represented by one of alternatively repeated "00" and "11", whereas "0" of a binary code is represented by either "01" or "10". Two bits of a CMI code indicating one bit of a binary code is called a "block". Clocks having the same period as the block period are used for decoding a CMI code into an original binary code. However, depending upon selection of either 0-phase clocks or .pi.-phase clocks at the initial condition, there may occur a failure of synchronization.
In order to solve the above-described problem, conventionally as disclosed in Japanese Patent Laid-open Publication JP-A-62-68336, the number of violating bits within one frame is counted, and if the count becomes equal to or larger than a setting value, then block asynchronization is considered to have occurred so that clocks are inverted, i.e., clocks 180 degrees shifted from presently used clocks are selected for preserving of synchronization.
Clocks are inverted upon generation of violating bits due to random errors of inputted CMI codes because such violating bits are considered to be block asynchronization even if block synchronization is being held. Such a case is regarded as an erroneous synchronization. A probability of the erroneous synchronization of a block is represented by P.sub.BERR. However, according to the above-described technique, such erroneous synchronization can be dealt with by properly determining the setting value, taking into consideration the following approximated equation of a block erroneous synchronization probability which is given by:
P.sub.BERR =.sub.N C.sub.K .times.(m/2.times.Pe).sup.K where N is the number of bits within a supervisory section, K is a setting value of violating bit number, m is a mark rate, Pe is a data error rate for binary codes, and C represents a combination in mathematics where ##EQU1##
The above technique, however, has not considered block synchronization preservation against burst errors of inputted CMI codes, resulting in erroneous synchronization. Further in such a case, it takes time corresponding to one supervisory section to recover synchronization, thus causing a lose of data.